There are currently three major research trends concerning the making of metal circuits via inkjet printing. Firstly, solutions containing metal nanoparticles may be used to print and form circuits after sintering. Secondly, a hybrid process that combines surface processing with spraying catalysts are employed, and results in metal circuits after electroless plating. Thirdly, photoresist patterns may be formed on copper clad laminates via lithography, and the laminates are etched with the photoresist patterns as the cover, thus forming metal circuits after the photoresist patterns are removed. The first method cannot be applied to flexible plastic substrates because such substrates cannot withstand the high temperature (200° C.-300° C.) necessary for sintering metal nanoparticles to form metal circuits. If laser was used to sinter circuits, production costs would be further increased due to the need for related facilities, and problems like imprecise and overlong sintering would make large-scale productions even more difficult. The third method is also inadequate for use with flexible plastic substrates, because such substrates are easily damaged by etching solutions and the acidic/basic solutions used in the lithography process. Therefore, the second method is crucial for preparing metal circuits on the flexible plastic substrates.
A method for forming metal patterns was disclosed in U.S. Patent No. US2005/015078, which includes the mixing of palladium ions with a polymer comprised of a hydrophilic polymer (like PVP) and an UV curable resin to result in an ink, and the use of a inkjet printing technique to form particular patterns onto specific areas of the substrate; the resin is then cured via UV light to form a pattern or an area that allows for deposition of a metal film; followed by the use of a reducing agent like DMBA (Dimethylamine borane) to reduce the palladium ions to palladium atoms; a metal film is finally deposited onto the pattern via electroless plating. This method is characterized in that a hydrophilic polymer like PVP is easily removed from an aqueous solution, so that the printed pattern is imparted with high porosity or highly branched tree-like structure under microscopic view. In other words, the total surface area of the pattern is greatly multiplied, and the plating reactivity of the pattern and the combination between the deposited metal layer and the pattern are significantly enhanced. However, the method requires an externally provided reducing agent and the steps thereof are more complicated.
Two previous literatures (N. R. Bieri, J. Chung, D. Poulikakos, Superlattices and Microstructures, 2004, 473-444, and S. H. Ko, J. Chung, H. Pan, Sensors and Actuators, 2006) had disclosed the use of a metal nanoparticle solution to print lead wires, and then obtaining a conductive metal circuit via laser sintering. The method is found to have following disadvantages as observed from the literatures: 1. the required facilities are expensive; 2. the time needed for making conductive circuits is long; 3. the production scale is small; 4. the metal layer obtained from sintering is thin and imperfect as well as prone to cause poor conductivity; 5. the control over alignment of sintering is difficult. Therefore, the method is not only highly complicated, but also not cost-effective.
A method for forming metal patterns is disclosed in U.S. Pat. Nos. 5,501,150 and 5,621,449, in which an ink containing a reducing agent and a compound of silver (such as silver nitrate) is printed on a substrate, and the substrate is heated under a high temperature for sintering and forming a metal pattern. However, the method easily harms common flexible plastic substrates, thus rendering the method inadequate for use with such substrates.